1. Field of the Invention
The present invention relates to a shut-off valve for use with injection molding apparatus. In particular, the shut-off valve is used to positively halt or stop material flow from an injection unit into the mold portion of the injection molding apparatus.
2. Summary of Prior Art
Injection molding-the process of injecting a quantity, or shot, of molten plastic into a moldxe2x80x94is today one of the world""s dominant forms of plastic article manufacture. In this process, plastic or polymer is made flowable by a combination of the application of heat and kinetic energy, usually through the use of a flighted screw that rotates and reciprocates within a heated containment barrel. A mold is designed and manufactured with a cavity configuration that allows for the specific part or parts to be molded. The mold is mounted in the injection molding machine clamp. Inside the mold on the upstream end is a sprue bushing. The flowable plastic then is injected, usually by action of the screw, into the mold cavities of a mold coupled to the injection unit.
The conventional coupling between the injection unit and the mold comprises a projection from the injection unit, commonly called the nozzle, which mates with a depression formed in a portion of the mold called the sprue bushing. Conventionally, the nozzle has a convex hemispherical shape that registers with a corresponding hemispherical depression in the sprue bushing. While this hemispherical arrangement is standard in the industry and lends a self-aligning feature to the coupling, it is not resistant to leaks and is the source of messy and possibly dangerous xe2x80x9cdroolingxe2x80x9d of molten plastic. The seal is dependent on proper alignment and properly machined hemispherical components. If either of these components is damaged or misaligned, leakage may occur. Leakage is common and many times this leaked plastic can catch on fire as many times the sprue bushing freezes up and must be heated with a torch.
Typically, the injection unit is mounted on an hydraulically actuated xe2x80x9csledxe2x80x9d that brings the nozzle into contact with the sprue bushing. The sled is provided to perform an operation known as xe2x80x9csprue breakxe2x80x9d that is not common now that heated runner systems are common in injection molds. Before heated sprues, frozen or solidified plastic would accumulate in the sprue bushing and nozzle. The sled was used to separate the sprue bushing and nozzle to break the frozen plastic xe2x80x9csprue.xe2x80x9d Most injection molding apparatus are still provided with an hydraulic or electric sled arrangement.
Before an injection molding process begins, it is common to purge the injection unit of air to insure a full shot of plastic into the mold. Commonly, this is done while the nozzle and sprue bushing are separated and the sprue bushing and platen (part of the mold assembly) are covered with cardboard to prevent plastic from solidifying or freezing over these components. Then, the nozzle is brought against the sprue bushing and injection may begin. Plastic from leakage and purging presents a fire hazard as well as being dangerous to nearby personnel.
In many injection molding applications, it is desirable to be able to positively halt or xe2x80x9cshut-offxe2x80x9d the flow of plastic from the injection unit into the mold tool. Conventionally, a valve is provided to accomplish this function that is called, logically, a xe2x80x9cshut-offxe2x80x9d valve. These valves typically are located between the injection unit and the mold itself.
Some conventional shut-off valves are operated by forced contact between the nozzle portion of the injection unit and the mold, that is, when the nozzle is pressed firmly against the mold tool, flow is permitted, when the nozzle pressure is released, flow is halted or shut-off. An improvement or variation on this arrangement is found in U.S. Pat. No. 3,719,310, Mar. 6, 1973, which provides a rather complex valving arrangement that permits a limited flow from the nozzle even when the nozzle is not in contact with the sprue bushing for purging of the injection unit.
Other shut-off valves use various mechanical, pneumatic, and hydraulic actuators to cycle the valve between permitting and obstructing flow between the injection unit and the mold. An example of a pneumatically operated shut-off valve is found in commonly owned U.S. Pat. No. 5,975,127.
A drawback to such valves is that they typically require extensive modification of existing injection molding apparatus. The modifications required typically serve no other purpose than to permit the use of a shut-off valve. Additionally, many of the valves require additional power sources for actuation, such as pressurized air or hydraulic fluid, which adds more equipment and maintenance to an already complicated assembly.
A need exists, therefore, for an improved coupling and shut-off valve that addresses the shortcomings found in the prior art.
It is a general object of the present invention to provide an improved coupling between an injection unit and mold of an injection molding apparatus that incorporates a shut-off feature that is positively actuated.
This and other objects of the present invention are achieved by providing an elongate projection from the injection unit. A passage extends through the projection in fluid communication with the injection unit. A generally cylindrical receptacle is formed in a portion of the mold for receiving the projection in generally concentric sliding relation. The receptacle is in fluid communication with the mold cavity, wherein relative movement between the projection and the receptacle causes injection unit to be selectively in fluid communication with the mold.
According to one embodiment of the preferred invention, the elongate projection includes a generally conical end portion.
According to one embodiment of the preferred invention, the receptacle has a generally conical opening.
According to one embodiment of the preferred invention, the elongate projection includes at least one port extending transversely and in communication with the passage.
According to one embodiment of the preferred invention, the receptacle is dimensioned to obstruct and seal the port in the projection against fluid flow.
According to one embodiment of the preferred invention, the receptacle includes an enlarged-diameter portion, wherein, upon alignment with between the projection and enlarged-diameter portion, the port in the projection is not obstructed and sealed against fluid flow.
According to one embodiment of the preferred invention, the receptacle includes a purge passage for selective alignment with the port in the projection.
According to one embodiment of the preferred invention, the relative movement between the projection and the receptacle occurs hydraulically by employing the hydraulic sled that moves the injection unit relative to the mold.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art with reference to the drawings and detailed description of the invention, which follow.